Preparation of nitrogen trichloride



Patented Sept. 30, 1 947 PREPARATION OF NITROGEN rnrcnnonmn Willis S. Hutchinson, St. Paul, Minn, assignor to General Mills, Inc., a corporation of Delaware No Drawing. Application February 1, 1943,

. Serial No. 474,408

The present invention relates to a process of preparing nitrogen trichloride in a manner which eliminates the corrosion problems of prior art methods.

Nitrogen trichloride is Widely used as ,a flour bleaching and maturing agent. It is applied in gaseous form at low partial pressures in admixture with air to flour particles agitated in a closed chamber in such a way that large surfaces of flour are exposed to the gas in order that the flour may be uniformly treated.

Prior commercial processes involve the reaction of chlorine or an aqueous solution of chlorine with an aqueous solution of an ammonium salt, such as ammonium chloride or ammonium suifate, followed by the subsequent stripping of the nitrogen trichloride from the solution by a stream of air. It is apparent that the corrosive solutions involved in such a process require special equipment which is not attacked under the conditions of treatment. The initial cost and upkeep of the equipment necessary to handle such corrosive materials and solutions and to handle the moist gas produced by these processes is very high. Special drain pipes capable of withstanding the hydrochloric or hydrochloric and sulfuric acids produced are necessary. In flour bleaching operations it is not uncommon to have the bleaching equipment on the fifth or sixth or even higher floor of a mill building. The amount of corrosion resistant drain pipe required was accordin ly great, as such protection is re- 11 Claims. (01. 23-205) quired from the equipment to the main sewer line. To place the bleaching equipment on a lowor floor would require a substantial alteration of the mill layout which would probably. cost more than the special corrosion resistant equipment.

Moreover, the chlorine supply to such generators frequently was controlled by'means of a stream of Water. Pressure fluctuations inthe water line at such elevations result'in an uneven chlorine input and consequently an uneven nitrogen trichloride output from the generators. Accordingly, uniformity in bleach was difficult to obtain.

The present invention provides a process of producing nitrogen trichloride which is cheaper, simpler and more accurate. Furthermore, it eliminates the use of corrosive solutions. As a result, one of the worst problems of the prior art processes is eliminated. Broadly, the invention involves the preparation of nitrogen trichloride by the reaction of chlorine on a solid ammonium salt, in the presence of sufiicient moisture to permit the reaction to proceed to substantial completion in a reasonable time period.

The reaction of chlorine with an ammonium salt to produce nitrogen trichloride proceeds more rapidly in the presence of some moisture. However, it i not necessary that suificient moisture be present so that the reactants are present as solutions. It has been found-that sufficient moisture for a rapid rate of reaction can be pres-- ent while the ammonium salts are in solid form.

This moisture maybe present in the reaction or, to add I and II equations, III is obtained 6C12+ (NI-I4) 2SO4+6HC1+2NC13 +H2SO4 III In the event that ammonium chloride is the salt used, only hydrochloric acid is formed.- In

some uses to which the nitrogen trichloride is put,

the acid which is formed during the generation, may be objectionable, particularly if it is relatively volatile as is the case with HCl. In such instances it may be desirable to provide some means of neutralizing the acid formed. This may be accomplished by providing in the reaction mixture a compound Which will neutralize the acid formed. When calcium carbonate is used, the reactions with hydrochloric and sulfuric acids are as follows:

By adding Equations III and IV, the following expression is obtained which illustrates the overall reaction involved.

2NC13/'+4GO2/' +CaSO4+3CaClz+4H2O V Instead of a neutralizing agent, it is possible to employ an absorbent or an adsorbent which will pick up and retain the acid.

It will be seen from the above equations that although Water enters into the reactions, none is consumed by the process. The moisture in Equation V is taken up by the calcium sulfate or the calcium chloride formed. In addition some of the moisture may be carried along with the stripping air. a

In order to increase the surface area of the ammonium salt so as to insure complete and uniform reaction of the chlorine, it is preferred to employ a bed composed of an inert carrier on which the ammonium salt is deposited. The inertcarrier should be porous and may either furnish or v retain moisture for reaction. Likewise it may be an absorbent or adsorbent to pick up acid formed. It may also be a neutralizing agent to dispose of the acid formed. Furthermore, the carrier may possess one or any combination of these properties or a mixture of carriers having any combination of propertiesmay be used. The,

following are examples of carriers which may be used: exfoliated vermiculite, granulatedIglass,

rock wool, granulated calcium carbonate, crushed neutralize the strong acid formed withoutneu tralizing any substantialquantity of hypochlorous acid, may be used.

The following is adetailed example of how the invention may. be employed. An ammonium salt may be mixed with aninert carrier such as is described above. The solid reactant material may be placed in a suitable container, such as a tower or a horizontal tube, and chlorine gas passed therethrough. In view of the explosive nature of nitrogen trichloride, the generation of this material should be performed at a sumciently low partial pressure to make, the operation safe. Such partial pressures as 25 to 50 mm., when, the gaseous mixture is substantially at atmospheric pressure, are suitable. This partial pressure may beobtained, by diluting the chlorine stream before contact with the ammonium salt or by introducing a separate stream of airinto the reaction chamber. The safe limit of NC13 concentration dependsupon the nature of the materials with which it is in contact. For example, in the presence of certain types of organic material or reducing agents, the above-mentioned partial pressure might possibly not be safe. Hence, organic materials are preierably avoided during the generation'when the partial pressure of C12 and N01; should be as high as is reasonably safe, in order to insure maximum capacity for a given size piece'of equipment. 'Afte'r 'generatiomthe partial pressure may be reduced for use, if desired or necessary. For instance, in flour bleaching, good results are secured with partial pressures of H013 in lairof 2 mm. Hg or less. It will be appreciated, however, that the maximum partial pressure which can be safely used; depends on the circumstances under which it is used.

The water for the process may be present in the solid reactant mixture or may besupplied in the air stream, or both. In View of the corrosive nature of wet chlorine, it is usually not desirable to introduce the moisture into the stream of chlorine, at least until the chlorine stream has:

been diluted with air or other inert gas.

The reaction betweenthe chlorineand the solid ammonium salt in the presence of moisture ispractically instantaneous, beingcomplete withm a few seconds. Whatever reaction time is necessary may be provided by suitably regulating the amount of ammonium salt and the flow of chlorine.

The process maybe carried voutin a batch manner or may be made continuous. In the batch operation a packed tower. may be used .until substantially exhausted, after which the equipment may be shut down and the exhausted ma terial replaced. In the continuous operation a number of packed towers may be provided, some oiwhich may be placed on stream while others are being repacked. In a further variation} a seriesof packed towers may be used in a countercurrent manner. That is, the chlorine stream a a. can h s ar y xha s e tower and then pass in sequence through less exhausted towers until the chlorine in the gas has been substantially completely converted to nitrogen trichloride. By means of suitable connections, an exhausted tower may be cut out of the streamv andthe chlorine directly introduced into the second tower. After the first tower has been replenished, it. maybe cut back into the stream as the'final tower. In this way complete utilization of the ammonium salt, as well as complete reaction of the chlorine, may be obtained.

Inthe previously described operations, control of the amount of nitrogen triohloride formed is most conveniently obtained by controlling the amount of chlorine put; into the process. Reliable means iormeteringandcontrolling the flow of chlorine are readily, available, andmay conveniently be applied to this process. As analternative, the ammonium salt may be usedas the controlling factor. In this operation suitable means for feedingsolid ammonium salt into a reaction chambermay be used to control the amount of nitrogen trichloride formed, In this operation, a slight excessof chlorine. may be em,- ployed to insure complete reaction with the ammonium salt. When the nitrogen trichloride is to be used for flour bleachin the slight, excess of chlorine is not disadvantageous, as it ef-' fects some additional bleaching.- At the same time, the loweringof thepI-I otthe flour may be beneficial if the amountof excess chlorine is kept within reascnablelimits.

Instead of employing packed towers for the reaction chamber it is possible to carry Out thepres ent reaction inany suitable type of conveyor, such as for example a screw-conyeymfr n that event,

the moistened salt mixture may befed intoa screw conveyor tube in quantities insufficient to. fill the tube and the chlorineegas passed, counter-current 7 to the flow of solid materiaL- The agitation produced by the conveyor increases the amount of tact mass co ntaining an acid neutralizer whichv will neutralize I-IQl butn tQHOCI, such as calcium carbonate. in accordance with EquationI reacts with the calcium, carbonate to form, a calcium. chloride which remains with the solid material; Thus the equilibrium of ,Equation,I ,-is,shiited.to favor the.

production of HOCL. The ,hypochlorous acid,

however, is too Weak to react with the calcium carbonate and as a result is vaporized and carried to another reaction chamber containingan ammonium salt wherein nitrogen trichloride, is formed. Insteadof forming hypochlorous acidin thismanner, it maybe obtained from any available sourceor generated in any. other suitable Way. The term chl orine as usedherein is intended to include these similar gases which function in a similanmannerin; the present process.

While various modifications of the, invention have been described, it;,- is un derst ood,that the same is not limited thereto,but may be varied within the scope of the appended claims.

I claim as my'invention" 1 1. A processgof preparing nftIiQ cn trichloride Thehvdrp hl r c acid .f rmed,

which comprises contacting gaseous .chlorine with a solid ammonium salt admixed with an inert carrier and in the presence of a small amount of moisture relative to the amount of solid ammonium salt.

2. A process of preparing nitrogen trichloride which comprises contacting gaseous chlorine with a solid ammonium salt admixed with a, carrier, said carrier being adapted to retain acid formed during the reaction, the process being carried out in the presence of a small amount of moisture relative to the amount of solid ammonium salt.

3. A process of preparing nitrogen trichloride which comprises contacting gaseous chlorine with a solid ammonium salt admixed with a carrier material adapted to neutralize acid formed during the reaction, the process being carried out in the presence of a small amount of moisture relative to the amount of solid ammonium salt.

4. A continuous process of producing nitrogen trichloride which comprises continuously producing a uniform stream of gaseous chlorine and continuously contacting said stream with a solid ammonium salt admixed with an inert carrier in the presence of a small amount of moisture as compared with the amount of solid ammonium salt, and continuously removing a stream of nitrogen trichloride from the reaction.

5. A process of producing nitrogen trichloride which comprises contacting gaseous chlorine with a bed containing a solid ammonium salt and an inert carrier rendering the bed porous, in the presence of a small amount of moisture as compared With the amount of solid ammonium salt. 6. A process of producing nitrogen trichloride which comprises contacting a solid ammonium salt with gaseous chlorine in the presence of a 8. A process of producing nitrogen trichloride which comprises contacting a solid ammonium salt admixed with an inert carrier with hypochlorous acid Vapors.

9. Process of preparing nitrogen trichloride which comprises contacting a solid ammonium salt admixed with an inert carrier with a gaseous mixture containing an inert gas and a relatively low partial pressure of chlorine, in the presence of a small amount of moisture relative to the amount of solid ammonium salt.

10. Process of preparing nitrogen trichloride which comprises preparing a mixture of air and gaseous chlorine, the latter being at a relatively low partial pressure, and passing said mixture through a tower containing a porous bed of a solid ammonium salt and an inert carrier in the presence of a small amount of moisture relative to the amount of solid ammonium salt.

11. Process of preparing nitrogen trichloride which comprises contacting a solid ammonium salt admixed with an inert carrier with a gas selected from the group consisting of chlorine and hypochlorous acid, the process being conducted in the presence of a small amount of moisture as compared with the amount of solid ammonium salt when the gaseous chlorine is used.

WILLIS S. HUTCHINSON.

REFERENCES CITED The following referencesare of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,328,484 Oosterhuis Aug. 31, 1943 1,510,132 Baker Sept. 30, 1924 1,378,644: Baker May 17, 1921 OTHER REFERENCES LIndustrie Chemique, July 1939, p. 45

Inorganic and Theoretical Chemistry, by Mellor, vol. 8 (1929), Longmans, Green & 00., New York, pp. 599 and 600. 

